Electronic Tunability of Ruthenium Formyl and Hydroxymethyl Intermediates Relevant to Sustainable CO-to-Methanol Conversion

Metal formyl and hydroxymethyl complexes are implicated
as key
intermediates in the (photo)electrochemical reduction of carbon oxides
(CO2 or CO) to liquid fuels, such as methanol. Formyl complexes,
and to a lesser extent hydroxymethyl complexes, have been previously
synthesized and characterized; nevertheless, the influence of electronic
modifications to ligands supporting these reactive carbon fragments
is not well understood. Herein, we report the synthesis of a family
of ruthenium polypyridyl carbonyl complexes of the form [Ru(4,4′-R,R-bpy)(tpy)(CO)]2+ bearing different substituents on the bipyridyl (bpy) ligand
(R = OMe, H, CF3). Treatment with NaBH4 as a
chemical reductant results in the formation of the formyl and subsequently
the hydroxymethyl and methyl complexes; each is characterized by comprehensive
NMR spectroscopy, mass spectrometry, and isotopic labeling studies.
An electron-donating modification (R = OMe) to the bpy ligand is shown
to significantly increase the lifetime of the formyl intermediate
and the yield of released methanol. We observe a clear linear dependence
of thermodynamic parameters on bpy electronics; however, the stability
of the formyl complex and the reactivity of the resulting hydroxymethyl
complex do not depend linearly on ligand electronics. We anticipate
that these results may be extended to future development of (photo)electrocatalytic
systems for CO-to-methanol conversion.